The invention relates to an electric incandescent lamp comprising a blown glass lamp vessel sealed in a vacuum-tight manner, having an axis of symmetry and a largest diameter transverse to the axis of symmetry. A neck-shaped first wall portion is behind the largest diameter and has a free end carrying a lamp cap having electric contacts. The lamp vessel has an internally concave second wall portion, and an internally concave wall portion which is substantially parabolically curved and which is a body of revolution of a parabola branch about the axis of symmetry, the parabola axis being remote from the axis of symmetry. An internally concave fourth wall portion is located opposite the lamp cap in front of the largest diameter. A helically wound filament is supported about the axis of symmetry substantially in a plane through the foci of the parabolically curved wall portion and at least partly coinciding with said foci, current-supply conductors interconnect the filament and contacts on the lamp cap.
The invention also relates to a blown glass bulb suitable for use in the lamp.
A lamp of this type is known from U.S. Pat. No. 2,110,590.
In the known lamp the parabolically curved wall portion is formed in such a way that its shape can be described as being produced by intersecting the parabola along its axis, spacing the halves apart from each other and subsequently jointly revolving them around the axis of symmetry. As a result, the lamp vessel is wider at the same curvature of the wall portion. The lamp vessel is elliptically curved from the plane through the foci located on a circle to the neck-shaped wall portion.
The known lamp provides a solution to the problem of parabolic reflectors being too narrow in the focal plane if the filament is to be mounted in a deep reflector so as to obtain a satisfactory beam of generated light without the transverse dimensions of the lamp exceeding a conventional size.
Although in the known lamp the branches of the parabola are moved apart in an axial cross-section, the filament is still in the focal plane in a narrow portion of the lamp vessel, far remote from the largest diameter of the lamp vessel. A drawback of the known lamp therefore is that only filaments consuming a relatively low power can be mounted in the lamp vessel in order to avoid overheating of the lamp vessel.
Electric lamps having a power value between 15 and 100 W, for example 15, 25, 40, 60, 75 and 100 W intended to be operated at a standard voltage are manufactured in a large number of types. The finish, the coating or the processing, of the lamp vessel wall, and also its shape and the size and shape of the inner parts of the lamp are different.
The electric incandescent lamps for operation at the mains voltage in the said power range include:
lamps such as those described in the above-cited U.S. Pat. No. 2,110,590, having a mirror-coated parabolically curved wall portion opposite to which a window is located. The window is glazed (is slightly light-scattering) for example due to an etching treatment and/or is colored;
lamps having a substantially spherical lamp bulb which is transparent or is frosted or which is coated with a white or colored light-scattering layer;
lamps having a conical wall portion adjacent to, and a curved wall portion opposite to the neck-shaped wall portion, the conical wall portion being provided with a white or coloured light-scattering layer and the curved wall portion being slightly light-scattering and possibly colored. These lamps emit light on all sides, but supply along the axis in directions remote from the neck-shaped wall portion a higher luminous intensity than in other directions;
lamps having opposite to the neck-shaped wall portion a spherical wall portion which is mirror-coated or is provided, for example with a white light-scattering coating.
The manufacture of this large number of lamp types is very complicated due to the variety of lamp vessel types which require on and between the production machinery their own supply and lead-out mechanisms and their own transport means, and which moreover require individual packaging. The readjustment of production machines from one lamp type to the other is thus a very laborious operation. Another complication of their manufacture is that the various types of lamps require different filament supports.